Replace Invalidation::Equals with Invalidation::operator==

This allows to rewrite SingleTopicInvalidationSet::operator==()
trivially, which as a side effect also eliminates undefined behavior.

Bug: 1287992
Test: Existing unit tests
Change-Id: I770e9dfcb828e00210f6ff725deeba3ea494c2ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3397925
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/main@{#960875}